One-piece induction seals typically include a membrane or foil layer combined with a heat seal layer for adhering the seal to an upper rim of a bottle or container. To support the foil and heat seal, such one-piece induction seals often include a top structural support or backing layer. The structural support is often a pulp, paper, paperboard, cardboard, chipboard, or other cellulose-type materials. This top backing layer provides a structural support for the barrier and heat seal layers of the induction seal.
In use, the one-piece induction seals are generally die-cut from sheets into a disk shape and, then, inserted into a container or bottle cap at a closure manufacture. The cap generally includes internal threading or other internal retention projections that hold the seal against the upper surface of the cap by friction or interference. The cap and seal combination may then be provided to an end user where the seal is induction sealed to the upper rim of a bottle or container when the cap is placed thereon after the container is filled with a product.
During periods of higher humidity, such as in the spring and summer months, the paper or pulp-based materials commonly used for the support layer tend to be susceptible to changes in the environment. When exposed to higher humidity levels, the paper or pulp-based materials tend to absorb moisture, swell, and then may deform or curl into a potato-chip like shape. If this backing curl is significant and occurs while the seal is in the cap or closure before being combined with the bottle or container, the threading or internal retention projections of the cap may not retain the seal within the cap. Due to the curling, the seal may actually fall out of the cap. In other cases, the curled seal may shift within the cap so that it can not be effectively induction sealed to the bottle during the bottle or container closing operations.